Method and apparatus for determining the position of a surgical tool relative to a target volume inside an animal body

ABSTRACT

A method for determining the position of a surgical tool relative to a target volume inside an animal body according to a pre-plan. The method includes the steps of i) obtaining a plurality of two-dimensional images of said target volume using an imaging device, each 2D-image being represented by an image data slice I(x,y,z); ii) reconstructing from said plurality of image data slices I(x,y,z) a three-dimensional image of said target volume using a transformation device, said 3D-image being represented by a volumetric image data array V(x,y,z); iii) displaying said three-dimensional image of said target volume to an user using a display.

This Non-provisional application claims priority under 35 U.S.C. §119(a)on patent application No(s). 03078243.7 filed in European Community onOct. 14, 2003, the entire contents of which are hereby incorporated byreference.

DESCRIPTION

The invention relates to a method for determining the position of asurgical tool relative to a target volume inside an animal bodyaccording to a pre-plan comprising the steps of

-   i) obtaining a plurality of two-dimensional images of said target    volume using imaging means, each 2D-image being represented by an    image data slice I(x,y,z);-   ii) reconstructing from said plurality of image data slices I(x,y,z)    a three-dimensional image of said target volume using transformation    means, said 3D-image being represented by a volumetric image data    array V(x,y,z);-   iii) displaying said three-dimensional image of said target volume    to an user using displaying means.

The invention furthermore relates to an apparatus for determining theposition of a surgical tool relative to a target volume inside an animalbody according to a pre-plan comprising

-   -   imaging means for obtaining a plurality of two-dimensional        images of said target volume, each 2D-image being represented by        an image data slice I(x,y,z);    -   transformation means for reconstructing from said plurality of        image data slices I(x,y,z) a three-dimensional image of said        target volume represented by a volumetric image data array        V(x,y,z);    -   storing means for storing said plurality of image data slices        I(x,y,z) and said volumetric image data array V(x,y,z);    -   displaying means for displaying said three-dimensional image of        said target volume to an user.

In the medical field, it is common to use imaging techniques to viewinternal organs of a subject. For example, in diagnosing prostatecancer, a diagnostician uses transrectal ultrasound (TRUS) to identifywhether lesions are present as well as to determine the location, sizeand extent of lesions if present. Conventional diagnostic imagingequipment based on the principle of ultrasound typically comprise anultrasound probe for transmitting ultrasound wave signals into thesubject and receiving reflected ultrasound wave signals therefrom. Thereflected ultrasound wave signals received by the ultrasound probe areprocessed and a two-dimensional image of the target volume underexamination is formed.

A typical embodiment of an ultrasound probe is an intracavitaryultrasound probe primarily employed in the fields of gynaecology andobstetrics for the purpose of examining intrapelvic organs, such as thevagina, the uterus and the ovaries by women.

Another application, wherein intracavitary ultrasound probes are used,concerns the treatment of prostate cancer by implanting radioactiveseeds through a hollow needle, which needle is inserted into the bodynear or in the prostate gland. An example of a device for effectingradiation therapy in an animal body by implanting radioactive seedsthrough a number of needles inserted in the animal body is for exampledisclosed in European patent application no. EP-A1-1 070 519. Prior toimplanting the radioactive seeds, in that device one or more hollowneedles are inserted into the animal body, wherein the exact location ofthe (tip of the) needle is monitored using images obtained with anintracavitary ultrasound probe, which probe is inserted into thepatient's rectum. The insertion of the needles towards their desiredpre-planned depth (location) is controlled using information obtainedfrom said images. Another imaging technique is based on the principle ofmagnetic resonance imaging (MRI).

The above imaging techniques generate two-dimensional image slices ofthe target volume of the patient's body. Multiple 2D image slices spacedapart from each other on one longitudinal direction are necessary toobtain an overall view of the internal organs (the target volume) of thepatient's body to be examined. There are several proposals to combinethese multiple 2D image slices and to transform them into athree-dimensional image resulting in an overall 3D view of the targetvolume being imaged.

A 2D to 3D conversion technique based on for example ultrasound imagingand according to the above preamble is disclosed in U.S. Pat. No.5,454,371.

It is an object of the invention to provide an improved method andapparatus utilizing a more sophisticated imaging technique to be used incombination with a pre-plan (for example a treatment plan).

According to the invention the method is characterized by the steps of

-   iv) selecting according to said pre-plan at least one specific    imaginary target location within said three-dimensional image being    displayed by said displaying means using selecting means;-   v) controlling said imaging means relative to said target volume for    obtaining in real time one two-dimensional image represented by an    image data slice I(x,y,z) of a specific target location within said    target volume corresponding to said specific imaginary target    location being selected within said three-dimensional image;-   vi) displaying said real time two-dimensional images of said    specific target location to the user using said displaying means;    and-   vii) determining the actual position of said surgical tool within    said specific target location using said real time two-dimensional    images of said specific target location being displayed.

With these features an improved imaging technique is realised, whereinthe diagnostician is capable in controlling the imaging means relativeto the target volume of the animal body to be examined/viewed. Thethree-dimensional image thus obtained from the two-dimensional imagesserves as an imaginary working space for the diagnostician. Throughmanipulation within said imaginary 3D working space displayed to thediagnostician, the latter can easily manipulate the imaging meansrelative to the patient's body in order to obtain a real time or nearreal time two-dimensional image of the region of interest of the targetvolume.

Said region of interest of the target volume has to be selected withinthe imaginary 3D image and based on said selection the method accordingto the invention automatically focusses the imaging means on said regionof interest of the target volume.

With this imaging technique according to the invention the diagnosticianis capable of easily tracing the presence and position of a surgicaltool relative to said target volume for example for treatment purposes.More in particular with this imaging technique the diagnostician is ableto monitor the course of movement of said surgical tool relative to saidtarget volume for example during insertion or navigation of saidsurgical tool through the animal body.

In a further improvement of the method according to the invention themethod further characterized by the steps of

-   viii) comparing said determined actual position of said surgical    tool with a pre-planned desired position of said surgical tool    relative to said target volume, and-   ix) correcting said determined actual position of said surgical tool    in view of said pre-planned desired position by repositioning said    surgical tool relative to said relative to a target volume.

Hence herewith the diagnostician is able to correct a sophisticated realtime manner the course of movement of said surgical tool relative tosaid target volume.

Especially when operating the imaging technique according to theinvention in combination with a certain pre-plan (for example apre-planned radiation treatment plan for treating prostate cancer withradioactive sources) the method can be advantageously furthercharacterized by the step of

-   x) monitoring said correction step ix) until the actual position of    said surgical tool corresponds with said pre-planned desired    position.

The diagnostician is further supported in performing said imaging methodaccording to the invention in combination with for example a pre-plan asthe method is further characterized by the steps of

-   xi) projecting according to said pre-plan during step iii) an    imaginary surgical tool within said three-dimensional image being    displayed, and-   xii) projecting within said three-dimensional image the actual    position of said surgical tool as determined with step vii).

This provides a simple, but advantageous feedback control in theimaginary 3D working space of the diagnostician as the latter isherewith continuously visionally informed about the exact position ofthe surgical tool relative to the target volume in relation to theintended or desired position as pre-planned.

Moreover the method according to the invention involves the step ofxiii) storing the image date obtained with steps i), ii) and/or v) usingstoring means.

The apparatus according to the invention is characterized by selectingmeans for selecting according to said pre-plan at least one specificimaginary target location within said three-dimensional image beingdisplayed by said displaying means; control means for controlling saidimaging means relative to said target volume for obtaining in real timeone two-dimensional image, represented by an image data slice I(x,y,z),of a specific target location within said target volume corresponding tosaid specific imaginary target location being selected within saidthree-dimensional image; and means for determining the actual positionof said surgical tool within said specific target location using saidreal time two-dimensional images of said specific target location.

When using the apparatus according to the invention controlling theimaging means relative to the target volume of the animal body to beexamined/viewed is allowed. Instead of directly controlling the imagingmeans relative to the target volume for example by direct manipulationof the imaging means in a specific orientation relative to the targetvolume, said manipulation is now performed in a remote manner. Thethree-dimensional image obtained from the two-dimensional images servesas an imaginary working space for the diagnostician.

Through manipulation within said imaginary -3D working space beingdisplayed, the diagnostician is able to orientate, to redirect as wellas to operate the imaging means relative to the patient's body in aremote manner in order to obtain a real time or near real timetwo-dimensional image of the region of interest of the target volume.

Direct operation of the imaging means is no longer necessary and an easytracing of the presence and/or position of a surgical tool relative tosaid target volume for example for treatment purposes is herewithpossible. More in particular with this imaging technique thediagnostician is able to monitor the course of movement of said surgicaltool relative to said target volume for example during insertion ornavigation of said surgical tool through the animal body.

In an improved embodiment of the apparatus according to the inventionallowing in a sophisticated real time manner the correction of thecourse of movement of said surgical tool relative to said target volumecomparison means are present for comparing said determined actualposition of said surgical tool with a desired position of said surgicaltool as pre-planned and also correcting means are present for correctingsaid determined actual position in view of said pre-planned desiredposition by repositioning said surgical tool relative to said relativeto a target volume. These features are especially suited whenimplementing the apparatus according to the invention in combinationwith a certain pre-plan (for example a pre-planned radiation treatmentplan for treating prostate cancer with radioactive sources).

More in particular an improved operation of the apparatus according tothe invention is obtained as displacement means are present fordisplacing said imaging means relative to said target volume based onsaid control means.

Especially said displacement means are capable of displacing saidimaging means in a longitudinal and/or a rotational direction resultingin an advantageous control of the imaging means in relation to thepre-plan.

As suitable imaging means ultrasound imaging means, for example a rectalultrasound imaging probe or magnetic nuclear imaging means can be usedwith the apparatus according to the invention.

When using magnetic resonance imaging the control means of the apparatusaccording to the invention are arranged for energizing the appropriategradient coils of the magnetic nuclear imaging means. This allows animproved control of the imaging means depending on the region ofinterest to be imaged as selected by the diagnostician in the imaginarythree-dimensional working space.

More in particular said selection means comprise a display pointer, amouse pointer or an input device, like a keyboard.

Various surgical tools can be used when operating the apparatusaccording to the invention. For example said surgical tool can be atleast one implant needle and/or at least one radiation emitting source,for example a radioactive brachytherapy seed or HDR source beinginserted through an implant needle. With these types of surgical toolsthe apparatus (and method) according to the invention as very suitablefor use in brachytherapy treatments (e.g. the treatment of prostatecancer).

The invention will now be described in combination with a drawing, whichdrawing shows in:

FIG. 1 shows in very schematic form a known radiation treatment deviceusing imaging means;

FIG. 2 a block diagram depicting a first embodiment of the methodaccording to the invention;

FIG. 3 a block diagram depicting a second embodiment of the methodaccording to the invention;

FIG. 4 a more detailed embodiment of an apparatus according to theinvention,

FIG. 5 a further more detailed embodiment of an apparatus according tothe invention,

FIG. 1 shows in very schematic form various elements of a knownradiation treatment device using a template-assembly for implanting oneor more energy emitting sources, e.g. radioactive seeds towards adesired location within an animal body, for example into a prostategland using ultrasound imaging means.

The known device shown in FIG. 1 operates as follows. A patient 1 isunder spinal or general anaesthesia and lies on the operating table 2 inlithotomy positions The (ultrasound) imaging probe 7 is introduced intothe rectum and the probe is connected via signal line 7 a with a wellknown image screen, where an image may be seen of the inside of thepatient in particular of the prostate gland 11 as seen from the point ofview of the imaging probe 7. The template-assembly 5 is attached to thestepping device 4, thereby insuring the correlation of the ultrasoundimage geometry and the template-assembly 5. Subsequently furtherneedles-10 are introduced in the body and the prostate gland underultrasound guidance one by one.

Moving the imaging probe with the drive means 4 longitudinally withinthe rectum controls the needle depths of each needle 10. After allneedles 10 have been placed, their positions relative to the prostategland 11 are determined in at least one of several known ways. In aknown way the therapy planning module 12 a determines how the needles 10are to be placed in the prostate and how many radiation emitting sourcesare to be placed in what order in each of the needles 10. Theinformation about the desired placement of the radioactive seeds in theneedles 10 is used to control the seed loading unit 8.

FIGS. 2 and 3 disclose a block diagram depicting a first and a-secondembodiment of the method according to the invention. In the Figures withreference numeral 20 imaging means are depicted, which in FIG. 2consists of imaging means based on magnetic resonance imaging. The MRImeans depicted with reference numeral 20 in FIG. 2 generally comprises apower source for energizing the gradient coils G_(x), G_(y), G_(z). Thegradient coils are in FIG. 2 depicted with reference numeral 21(G_(x)−G_(y)−G_(z)).

When independently energized, the three gradient coils produce alinearly variable magnetic field in any direction, where the netgradient is equal to √{square root over ((G_(x) ²+G_(y) ²+G_(z) ²))}.With an appropriate design, the gradient coils G_(x), G_(y), G_(z)create a magnetic field, that linearly varies in strength versusdistance over a predefined field of view. When superimposed upon ahomogeneous magnetic field B_(o) not shown in the figures positivefields add to B_(o) and negative gradient fields reduce B_(o).

The resulting gradient is linear, position-dependent and it causesprotons to alter their precessional frequency corresponding to theirposition along the applied gradient in a known and predictable way. Anygradient direction is possible by superimposition of the three fields.MRI involves RF excitations at the Larmor-frequency of the protonscombined with magnetic field gradients to localize the signal from eachindividual volume element after the excitation.

The MR imaging means 20 are connected to a central processing unit CPU22 intended for operating the imaging means 20.

Central processing unit 22 interacts with transformation means 23, whichmeans include digitizer and image processing units for digitizing andfurther processing the two-dimensional images generated by said imagingmeans 20.

Hereto the transformation means 23 may include storage means (forexample physical memory) for storing the two-dimensional images obtainedfrom the imaging means 20 as 2D information. The transformation means 23capture and digitize the two-dimensional information transfers the 2Dinformation to a computer system 24 for planning and visualisationpurposes. Also in computer system 24 said two-dimensional imageinformation is transformed into a three-dimensional image of the targetvolume being imaged.

Said computer system 24 may include planning software for pre-planningfor example a radioactive treatment on the target volume being imaged byimaging means 20.

As will be elucidated in more detail with reference to FIG. 4 the methodaccording to the invention is further characterized in that within saidthree-dimensional image being visualized in said computer system 24 aspecific imaginary target location is to be selected using suitableselecting means. Upon said selection in said computer means 24 thecentral processing unit 22 is provided with control signals from saidcomputer system 24, which CPU 22 in turn controls the MR imaging means20 for obtaining a two-dimensional image in which image the targetlocation within the target volume corresponding with the imaginarytarget location selected within said three-dimensional image isobtained.

In FIG. 3 the method according to the invention is described using anultrasound probe 20 having an ultrasound transducer 21 which mayconsists of multiple ultrasound transducer elements 21 _(n). Theultrasound probe 20 is provided with drive means (not shown) fordisplacing the transducer element 21 in longitudinal direction or inrotational direction relative to the target volume to be imaged.

Ultrasound wave signals emitted by the transducer element 21 towards thetarget volume to be examined are orientated in a physical interactionfield that intersects the target volume to be imaged as a slice. Withinsaid physical interaction field the ultrasound wave signals can betransmitted, absorbed or reflected dependent on the composition of thetissue.

The reflected ultrasound wave signals are received by the transducerelement 21 and fed to an ultrasound processing unit (transformationmeans) 23 for generating a two-dimensional image corresponding to thephysical interaction field (image slice) of the target volume. Byrotating the transducer element 21 multiple two-dimensional image slicesspaced apart from each other are obtained.

As in FIG. 2, also in FIG. 3 unit 24 transforms these two-dimensionalimage slices into a three-dimensional image, which is used by saidcomputer system 24 for planning and visualisation purposes.

By selecting an imaginary target location within said three-dimensionalimage being displayed by said computer system 24 control signals aregenerated and fed to the control means of the ultrasound probe 20. Basedon said control signals the transducer element 21 is displaced inlongitudinal or rotational orientation relative to the target volumesuch that the physical interaction field of the ultrasound wave signalspropagating towards the target volume corresponds with the imaginarytarget location selected within said three-dimensional image.

Hence one two-dimensional image is obtained of said specific targetlocation within the target volume corresponding with the imaginarytarget location selected within said three-dimensional image.

In FIG. 4 a more detailed embodiment of an apparatus implementing themethod according to the invention is disclosed. For a properunderstanding likewise parts are described with identical referencenumerals.

In FIG. 4 reference numeral 30 depicts schematically an animal body (apatient), whereas reference numeral 31 depicts the target volume underexamination, for example an internal organ contained in said animal body30. Referring to FIG. 1 said organ 31 can be the prostate gland of amale person 30, which person has to undergo a radioactive therapytreatment session by implanting—using multiple implant needles 28_(n)—one or more energy emitting sources, for example radioactive seeds,through said implant needles 28 _(n) at desired locations within theprostate 31.

For performing the imaging technique as used with the method andapparatus according to the invention magnetic resonance imaging means 20(MRI) are used. As already described in FIG. 2 each MRI means comprisemultiple gradient coils 21 (G_(x), G_(y), G_(z)), which coils can beenergized or activated separately depending on the image acquisition tobe performed.

The method and apparatus according to the invention are based on theimaging technique to obtain a three-dimensional image of a specifictarget volume 31 inside an animal body 30 by generating a plurality oftwo-dimensional image slices 31 _(2D) with a proper operation of theimaging means 20.

For obtaining multiple two-dimensional image slices 31 _(2D) thegradient coils 21 (G_(x), G_(y), G_(z)) are controlled in a such mannerthat a physical interaction field intersecting or slicing the targetvolume 31 corresponding with an image slice is created.

The magnetic field and RF created by said magnetic resonance imagingmeans interact with the tissue of the target volume 31 in said physicalinteraction field. The interaction between the magnetic field generatedby the gradient coils 21 (G_(x), G_(y), G_(z)) is collected by areceiver coil 25 of the magnetic resonance imaging means 20 resulting ina two-dimensional image corresponding with the visual representation ofthe target volume 31 within said slice (physical interaction field).

A plurality of two-dimensional image slices 31 _(2D), are collected andstored in suitable storage means in a processing unit 33. The pluralityof two-dimensional image slices 31 _(2D) are transformed by computersystem 24 using suitable transformation means into a three-dimensionalimage of the target volume 31. The three-dimensional image is storedwithin computer system 24 and displayed on a display 24 a (referencenumeral 31 _(3D)).

This transformation technique for generating a three-dimensional imagefrom a plurality of two-dimensional image slices is known in the art.

According to the invention it is intended to manipulate the imagingmeans 20 by selecting a specific imaginary target location 37 within thethree-dimensional image 31 _(3D) being displayed on display 24 a. Thetarget location being selected is in FIG. 4 depicted with a circle 37.The selection of said imaginary target location 37 can be performedusing specific selection means like for example a mouse pointer adisplay pointer (for example a light pen) or an other suitable inputdevice like a computer keyboard.

The selection of the specific imaginary target location or region 37triggers or activates control means 22 (central processing unit) forcontrolling the imaging means 20. In FIG. 4, due to the selection ofspecific target region 37 within the imaginary three-dimensional image31 _(3D) the magnetic resonance imaging means 20 arecontrolled/activated such that the gradient coils 21 (G_(x), G_(y),G_(z)) are activated such, that one two-dimensional image 31 _(2D)′ ofsaid imaginary target region 37 as selected is obtained viatransformation means 23 where said single two-dimension image slice isimaged and digitized for further displaying purposes.

This digitized two-dimensional image 31 _(2D)′ is displayed to the user(diagnostician) using display means 34. Said display means 34 can be aseparate display device being part of transformation means.23, but itcan also be implemented in the display 24 b of the computer system 24.In the latter case display 24 b is divided in several sub-windows,wherein each sub-window is used for displaying the three-dimensionalimage 31 _(3D) or the single two-dimensional image 31 _(2D) of thespecific target region 37 as selected in said three-dimensional image 31_(3D) respectively.

In a likewise manner is it possible to implement the method andapparatus in combination with ultrasound imaging means as depicted inFIG. 5. In this FIG. 5 ultrasound image slice are produced using thetransducer elements 21, of the ultrasound probe 20. Said probe 20is—like in FIG. 1—inserted into the rectum of a male patient 30 forultrasound imaging of the internal organs and more in particular theprostate gland 31.

Ultrasound wave signals 26 are transmitted by the transducer elements 21n (of ultrasound transducer 21) towards the prostate gland 31 (targetvolume) and reflected ultrasound wave signals are received therefrom.The reflected ultrasound wave signals received by the ultrasound probe20 are processed by the ultrasound processing unit (transformationmeans) 23 and a two-dimensional image of the target volume underexamination is formed. By rotating the ultrasound probe 20 (theultrasound transducer 21) relative to the target volume subsequenttwo-dimensional image slices 31 _(2D) are obtained and processed byultrasound processing unit (transformation means) 23.

The transformation means 23 capture and digitize the two-dimensionalimage information and transfers the 2D information to a computer system24 for planning and visualisation purposes. Also in computer system 24said plurality of two-dimensional image slices 31 _(2D) are transformedinto a three-dimensional image 31 _(3D) of the target volume 31 beingimaged. Said computer system 24 may include planning software forpre-planning for example a radioactive treatment on the target volume 31being imaged by imaging means 20.

Similar to the embodiment of FIG. 4 the ultrasound imaging probe 20 canbe manipulated by selecting a specific imaginary target location 37within the three-dimensional image 31 _(3D) being displayed on display24 a. The target location being selected is in FIG. 5 depicted with acircle 37.

Due to the selection of the specific imaginary target location or region37 the ultrasound probe 20 is controlled by computer system 24. Theultrasound transducer 21 can be displaced in longitudinal and rotationmanner relative to the target volume 31 in order to create a psychicalinteraction field through the target volume 31 corresponding to theselected imaginary target location 37. The re-orientation of theultrasound transducer 21 relative to the target volume generates onetwo-dimensional image 31 _(2D)′ of said imaginary target region 37 asselected. Said single two-dimensional image 31 _(2D)′ is captured anddigitized by transformation means 23 for further displaying purposes.

This digitized two-dimensional image 31 _(2D)′ is displayed to the user(diagnostician) using display means 34. Said display means 34 can be aseparate display device being part of transformation means.23, but itcan also be implemented in the display 24 b of the computer system 24.In the latter case display 24 b is divided in several sub-windows,wherein each sub-window is used for displaying the three-dimensionalimage 31 _(3D) or the single two-dimensional image 31 _(2D)′ of thespecific target region 37 as selected in said three-dimensional image 31_(3D) respectively.

Hence with the method and apparatus according to the invention it ispossible to control the imaging means 20 (MRI or ultrasound) in anindirect remote manner by selecting an image direction in thethree-dimensional image of the target volume 31 within the animal body30. An user can control the magnetic resonance imaging means 20 frombehind display 24 a using the computer system 24 and control means 22.

This imaging technique is beneficial for example medical applicationsusing a pre-plan, for example a pre-planned therapy treatment and morein particularly for use with the brachytherapy treatment of prostatecancer using the device as depicted in FIG. 1.

For a treatment of prostate cancer using multiple implant needles 28_(n) to be inserted inside a prostate gland 31 of a male person 30 thedesired, pre-planned location/depth of the multiple implant needles 28_(n) is pre-planned according to a desired treatment therapy using knowntreatment planning software contained for example in computer system 24.The implant needles 28 _(n) are considered a surgical tool, and aredisplayed as imaginary surgical tools 28 _(n)′ and projected on saidthree-dimensional image 31 _(3D) of the prostate gland 31.

Likewise it is possible to project a frame 24 b of vertical andhorizontal lines on said three-dimensional image 31 _(3D), which frame24 b may correspond with aperture or grid orientation ontemplate-assembly 5 as depicted in FIG. 1. By selecting a specific gridposition within frame 24 b a specific implant needle 28 ₁ (for examplethe implant needle depicted with reference numeral 29) is selected andits insertion through the animal body 30 towards its desired,pre-planned depth within the prostate gland 31 can be monitored with thesingle two-dimensional image 31 _(2D)′.

The selection of the grid position within frame 24 b corresponding withspecific implant needle 29 leads to a remote control of imaging means 20via control means 22 resulting in one two-dimensional image 31 _(2D)′depicting the image slice intersecting with said grid position.

As with this imaging technique the insertion of the specific implantneedle 29 can be monitored in real-time it is possible to control viacontrol line 35 the needle insertion means 36 until said implant needle29 reaches its desired, pre-planned depth relative to the prostate gland31.

Subsequent energy emitting sources, for example radioactive seeds, canbe inserted through said implant needle 29 for performing a radioactivetherapy treatment as pre-planned using planning software contained incomputer system 24.

1. A method for determining the position of a target location associatedwith a portion of a surgical tool in an animal body according to apre-plan specifying a desired position of the surgical tool, the methodcomprising: obtaining a plurality of two-dimensional images in real timeof a target volume using an ultrasound imaging means, eachtwo-dimensional image being represented by a respective image dataslice, wherein the plurality of two-dimensional data slices are spacedapart from each other in one longitudinal direction; transforming saidplurality of two-dimensional data slices into a three-dimensional imageof the target volume represented by a volumetric image data array;displaying the three-dimensional image of the target volume using adisplay means; receiving a selection, via the display means, of at leastone imaginary target location defining a desired image view of at leasta portion of the surgical tool within said three-dimensional image;determining, based on said imaginary target location, a control signalfor displacing said imaging means to obtain a two-dimensional image ofthe imaginary target location; displacing said imaging means relative tosaid target volume, based on said control signal, for obtaining a realtime two-dimensional image of the imaginary target location; anddetermining the actual position of the surgical tool in the real timetwo-dimensional image which includes the imaginary target location. 2.The method according to claim 1, further including: correcting saiddetermined actual position of said surgical tool in view of said desiredposition by repositioning said surgical tool relative to said targetvolume.
 3. The method according to claim 2, further including:monitoring said correction of said determined actual position until thecorrected actual position corresponds with said desired position of thesurgical tool.
 4. The method according to claim 1, further including:displaying the three-dimensional image of the target volume on thedisplay means by including the actual position determined of the targetlocation; and displaying within said three-dimensional image the desiredposition of said surgical tool according to the pre-plan.
 5. Anapparatus for determining the position of a target location associatedwith a surgical tool inside an animal body according to a pre-planspecifying a desired position of the surgical tool, the apparatuscomprising: ultrasound imaging means for obtaining a plurality oftwo-dimensional images of a target volume, each two-dimensional imagebeing represented by a respective image data slice, wherein theplurality of image data slices are spaced from each other in alongitudinal direction; transformation means for reconstructing, fromsaid plurality of image data slices, a three-dimensional image of saidtarget volume represented by a volumetric image data array; storingmeans for storing said plurality of image data slices and saidvolumetric image data array; displaying means for displaying saidthree-dimensional image of said target volume to a user; selecting meansfor selecting, via the displaying means, at least one specific imaginarytarget region defining a desired image view of at least a portion of thesurgical tool within said three-dimensional image displayed by saiddisplaying means; means for determining, based on said imaginary targetlocation, a control signal for displacing said imaging means to obtain atwo-dimensional image of the imaginary target location; displacementmeans for displacing said imaging means relative to said target volumebased on said control signal for obtaining a real time two-dimensionalimage of the imaginary target location within said target volume; andmeans for determining the actual position of said surgical tool in thereal time two-dimensional image including the imaginary target location.6. The apparatus according to claim 5, further comprising: comparisonmeans for comparing said determined actual position of said surgicaltool with a desired position of said surgical tool as pre-planned; andcorrecting means for correcting said determined actual position in viewof said desired position by repositioning said surgical tool relative tosaid target volume.
 7. The apparatus according to claim 5, wherein saiddisplacement means is configured to displace said imaging means in atleast one of a longitudinal or a rotational direction.
 8. The apparatusaccording to claim 5, wherein said imaging means is a rectal ultrasoundimaging probe.
 9. The apparatus according to claim 5, wherein theselecting means comprise at least one of a display pointer, a mousepointer and an input device.
 10. The apparatus according to claim 5,wherein said surgical tool includes at least one implant needle.
 11. Theapparatus according to claim 5, wherein said surgical tool includes atleast one of a radioactive brachytherapy seed or an HDR source.
 12. Theapparatus of claim 5, further comprising: means for monitoring insertionof a specific implant needle in real-time; means for controlling, via acontrol line, a needle insertion means until the implant needle reachesa pre-planned depth relative to a prostate gland; and means forinserting radioactive seeds through the implant needle for performing aradioactive therapy treatment.